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. Author manuscript; available in PMC: 2011 Dec 1.
Published in final edited form as: J Am Chem Soc. 2010 Nov 5;132(47):16873–16882. doi: 10.1021/ja106251f

A Facile Approach Towards Multicomponent Supramolecular Structures: Selective Self-Assembly via Charge Separation

Yao-Rong Zheng 1,*, Zhigang Zhao 1, Ming Wang 1, Koushik Ghosh 1, J Bryant Pollock 1, Timothy R Cook 1, Peter J Stang 1,*
PMCID: PMC3016897  NIHMSID: NIHMS251001  PMID: 21053935

Abstract

A novel approach towards the construction of multicomponent two-dimensional (2-D) and three-dimensional (3-D) metallosupramolecules is reported. Simply by mixing carboxylate and pyridyl ligands with cis-Pt(PEt3)2(OTf)2 in a proper ratio, coordination-driven self-assembly occurs, allowing for selective generation of discrete multicomponent structures via charge separation on the metal centers. Using this method, a variety of 2-D rectangles and 3-D prisms were prepared under mild conditions. Moreover, multicomponent self-assembly can also be achieved by supramolecule-to-supramolecule transformations. The products were characterized by 31P and 1H multinuclear NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and pulsed-field-gradient spin echo (PGSE) NMR techniques together with computational simulations.

Keywords: Multicomponent Supramolecular Structures, Selective Self-Assembly, Coordination-Driven Self-Assembly, Supramolecular Rectangle, Supramolecular Prisms, Supramolecular Transformation

Introduction

Over the last decade, coordination-driven self-assembly has evolved into a well-established methodology for constructing novel metallosupramolecular structures using dative metal-ligand bonding interactions. A variety of elaborate metallosupramolecules, from two-dimensional (2-D) polygons to three-dimensional (3-D) cages, prisms, and polyhedra, have been reported based on coordination-driven self-assembly.1 By virtue of the resulting well-defined structures, coordination-driven self-assembly has a wide range of applications, such as synthesis of metallosupramolecular dendrimers,2 encapsulation of guests,3 and uses in catalysis4 and sensors.5 However, the design of coordination-driven self-assembly has been mostly constrained to two-component systems, in which only one metallic and one organic component is used.1 The two-component approach endows coordination-driven self-assembly with easy control and design, but significantly limits the versatility of molecular components involved, and, therefore, the diversity of the resulting supramolecules.6 In order to broaden the diversity of coordination-driven self-assembly, it is essential to explore controlled self-assembly within a multicomponent system.

Multicomponent, selective self-assembly represents a unique assembly process, in which multiple varying components can selectively recognize and combine to generate only one discrete structure within a mixture.7 Multicomponent selective self-assembly is a critical phenomenon in many biological systems. Viral capsids such as Tomato Bushy Stunt Virus and Rhinovirus, for example, are assembled by three and four different subunits.8 The proteasome of yeast Saccharomyces Cerevisiae is constructed from pairs of seven different proteins.9 However, to obtain multicomponent selective self-assembly in an abiological system is a formidable challenge. By mixing various molecular components that lack sufficient complementary electronic and/or structural information, a self-organized mixture or even disordered oligomeric species can be formed instead of one finite, discrete supramolecule.10 How to provide sufficient molecular information to control selective self-assembly in a multicomponent system remains a demanding issue in modern supramolecular chemistry.6

In the area of coordination-driven self-assembly, several methods have been developed to achieve multicomponent, selective self-assembly. Sauvage11 and Lehn,12 in a pioneering study, used topological information to guide the selective self-assembly of multicomponent pseudorotaxanes. Recently, it was found that steric constraint could be exploited to control the multicomponent selective self-assembly, as evidenced by the impressive examples of Schmittel,13 Fujita,14 and Kobayashi.15 However, the incorporation of topological or steric information into molecular components requires significant synthetic effort. Fujita demonstrated the facile selective self-assembly of 3-D trigonal prisms by mixing Palladium (II) acceptors with di- and trioptic pyridyl ligands, but a template is essential for the self-assembly.16

Recently, we demonstrated the facile selective self-assembly of multicomponent 2-D fused polygons and 3-D tetragonal prisms,17 and both were achieved just by mixing suitable directional organoplatinum acceptors and different pyridyl donors without templates. These assemblies rely mainly on the stoichiometry and directionality of the molecular components, and are controlled by maximum site occupancy and entropy. To further extend the scope of facile, selective self-assembly and the diversity of multicomponent coordination-driven self-assembly, herein we present a multicomponent approach using the coordination-driven self-assembly of a 90° Pt(II) acceptor with pyridyl and carboxylate ligands. With suitable stoichiometry and geometries of the molecular binding units, 2-D supramolecular rectangles (Scheme 1) and 3-D prisms (Scheme 2) can be obtained selectively from multicomponent mixtures, due to the lower energy of the heteroleptic system, as shown in Scheme 3. The products were characterized via 31P and 1H multinuclear NMR spectroscopy, electrospray ionization (ESI) mass spectrometry, and pulsed-field-gradient spin-echo (PGSE) NMR measurements together with computational simulations. In addition, a porphyrin tetragonal prism constructed by this method demonstrated the ability to encapsulate triphenylene in an aqueous acetone solution.

Scheme 1.

Scheme 1

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Selective self-assembly of a multicomponent rectangle 4 by the combination of cis-Pt(PEt3)2(OTf)2 1, dicarboxylate ligand 2, and linear dipyridyl donor 3.

Scheme 2.

Scheme 2

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Selective self-assembly of multicomponent trigonal (7) and tetragonal (8) prisms by combination of cis-Pt(PEt3)2(OTf)2 1, dicarboxylate ligand 2, and tritopic (5) and tetratopic (6) pyridyl donors.

Scheme 3.

Scheme 3

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Representation of selective self-assembly of cis-Pt(PEt3)2(OTf)2 with carboxylate and pyridyl moieties due to the lower energy of the heteroleptic System.

Furthermore, this selective self-assembly can be achieved not only by the assembly of the individual molecular components, but also via supramolecule-to-supramolecule transformations. Supramolecular transformations have been reported that rely on triggering of the molecular subunits by light, solvent variation, or chemical signals,18 but this phenomenon is rarely observed in a selective self-assembly system.19 We investigated supramolecular transformations in multicomponent selective self-assembly as shown in Scheme 4, where 90° Pt(II) acceptors and pyridyl ligands self-assemble into well-defined homoleptic6 supramolecular structures. Addition of a neutral triangle, assembled by the 90° Pt(II) acceptor and the carboxylate ligand, to the pyridyl-based square results in conversion of the homoleptic structures into a single, multicomponent heteroleptic6 supramolecule of different topology. These supramolecular transformation processes may be monitored by 31P and 1H multinuclear NMR spectroscopy.

Scheme 4.

Scheme 4

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Supramolecular transformations of square 9, truncated tetrahedron 10, and trigonal prism 11 into rectangle 4, trigonal prism 7, and tetragonal prism 8b, respectively, upon addition of the neutral triangle 12 assembled by cis-Pt(PEt3)2(OTf)2 1 and carboxylate ligand 2.

Results and Discussions

Selective self-assembly of a multicomponent supramolecular rectangle

Cis-Pt(PEt3)2(OTf)2 1 was mixed with dicarboxylate ligand 2 and linear dipyridyl donor 3 in a 2:1:1 ratio, followed by addition of D2O and Acetone-d6. After 3h of heating at 75 °C, all solvent was removed from the clear solution, and Acetone-d6 was then added into the mixture. A clear solution was afforded after an additional 5 h of heating at 75 °C, containing the supramolecular rectangle 4 as shown in Scheme 1. 31P and 1H multinuclear NMR spectroscopy and ESI mass spectrometry were used to characterize 4.

In the 31P{1H} NMR spectrum (Figure 1b), two coupled doublets at 6.60 ppm and 1.06 ppm (2JP-P = 22.0 Hz ) of approximately equal intensity with concomitant 195Pt satellites were found, indicating that the Pt(II) centers of 4 bear a heteroleptic coordination motif with pyridyl and carboxylate moieties.20 The doublet at 1.06 ppm is shifted approximately 12 ppm upfield relative to 1 (Figure 1a) upon coordination, and corresponds to the phosphorous nuclei trans to the pyridine ring, while the doublet at 6.60 ppm is due to the phosphorous nuclei opposite to the carboxylate group.20a–c The two signals are coupled, indicating that chemically inequivalent phosphorous nuclei are bound to the same Pt(II) center, consistent with the heteroleptic coordination motif of rectangle 4. In the 1H NMR spectrum (Figure S1 in the Supporting Information), signals corresponding to the coordinated pyridine and carboxylate ligands were identified at 9.00 ppm (Hα-Py), 7.74 ppm (Hβ-Py), and 7.66 ppm (Hphenyl). The sharp and identifiable signals in both the 31P{1H} and 1H NMR spectra support the self-assembly of the highly symmetric rectangle 4 as the predominant product in the mixture, and rule out the formation homoleptic assemblies or oligomers. ESI mass spectrometry further confirms the formation of a [4+2+2] multicomponent supramolecular rectangle 4. In Figure 2, peaks attributable to 4 with lose of two and three triflate anions can be observed at m/z = 1455.69 ([M – 2OTf]2+) and m/z = 920.88 ([M – 3OTf]3+). All these peaks are isotopically resolved and in good agreement with the theoretical distribution.

Figure 1.

Figure 1

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31P{1H} NMR spectra of cis-Pt(PEt3)2(OTf)2 1 (a) and the multicomponent supramolecular rectangle 4 (b).

Figure 2.

Figure 2

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Full ESI mass spectrum of the solution of the multicomponent supramolecular rectangle 4.

Selective self-assembly of multicomponent supramolecular prisms

To date, the selective self-assembly of 3-D supramolecules have always required a template,16 and are rarely obtained based solely on the intrinsic information of the complementary subunits.13 Herein, we report the selective self-assembly of 3-D supramolecular prisms by mixing a 90° Pt(II) acceptor, a carboxylate ligand, and different multi-pyridyl ligands, as shown in Scheme 2.

Upon mixing 90° Pt(II) acceptor 1 and carboxylate ligand 2 with tri- or tetrapyridyl donors 5 or 6 in a specific ratio (for 7: 1:2:5 = 6:3:2; for 8: 1:2:6 = 8:4:2), supramolecular prisms 7 and 8 were formed as the predominant species, after equilibration. The 31P{1H} NMR spectra (Figure 3) of 7 and 8 are dominated by two coupled doublets (7: 6.56 ppm and 1.01 ppm, 2JP-P = 22.0 Hz; 8a: 5.88 ppm and 1.08 ppm, 2JP-P = 21.4 Hz; 8b: 5.07 ppm and −0.34 ppm, 2JP-P = 21.4 Hz) of similar intensity with concomitant 195Pt satellites. These data, as expected, support the heteroleptic coordination environments of supramolecular prisms 7 and 8, and rule out the formation of homoleptic complexes or oligomers. Likewise, in the 1H NMR spectra (Figure S2-4 in Supporting Information), signals attributable to the coordinated pyridyl and carboxylate moieties are observed for 7: 9.02 ppm (Hα-Py), 7.75 ppm (Hβ-Py), 7.77 ppm (Hphenyl); for 8a: 8.87 ppm (Hα-Py), 7.89 ppm (Hβ-Py), 7.60 ppm (Hphenyl); and for 8b: 9.27 ppm (Hα-Py), 8.34 ppm (Hβ-Py), 8.09 ppm (Hphenyl). The sharp signals in both the 31P{1H} and 1H NMR spectra support the predominant formation of highly symmetric supramolecules 7 and 8.

Figure 3.

Figure 3

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31P{1H} NMR spectra of the trigonal prism 7 (a) and tetragonal prisms 8a (b) and 8b (c). ESI mass spectral data further support the self-assembly of supramolecular prisms 7 and 8. As shown in Figure 4 and Figure S5,6 (see Supporting Information), intense ESI mass peaks corresponding to consecutive loss of triflate anions from trigonal prism 7: m/z = 2218.76 [M – 2OTf]2+ and m/z = 1429.59 [M – 3OTf]3+ were observed, as were those corresponding to the tetragonal prisms: 8a at m/z = 2037.75 [M – 3PF6]3+ and m/z = 1164.50 [M – 5PF6]6+ and 8b at m/z = 2022.71 [M – 3PF6]3+ and m/z = 1155.62 [M – 5PF6]5+. All of these peaks are isotopically resolved and agree well with their theoretical distributions.

While suitable X-ray-quality crystals were not obtained, a computational study (see Supporting Information) together with PGSE NMR measurements was carried out to gain insight into the structural parameters of these assemblies.21 A molecular dynamics simulation using Maestro and Macromodel with a MMFF or MM2* force field, at 300K, in the gas phase was applied to equilibrate each supramolecule, and the output of the simulation was then minimized to full convergence. As shown in Figure 5, models of assemblies 7 and 8 have the shape of a trigonal and tetragonal prisms, respectively, with radii of 12 ε (7), 13 ε (8a), and 12 ε (8b). PGSE NMR experiments were carried out to measure the hydrodynamic radius for these assemblies and the results from these measurements agree with those from the modeled structures: 12.1 ± 0.1 ε (7), 11.6 ± 0.3 ε (8a), and 10.9 ± 0.1 ε (8b).

Figure 5.

Figure 5

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Computational simulations of trigonal prism 7 (a) and tetragonal prisms 8a (b) and 8b (c).

The multinuclear (31P and 1H) NMR spectroscopy, ESI mass spectrometry, and PGSE NMR measurements, along with the computational simulations, clearly support the selective self-assembly of heteroleptic 2-D and 3-D multicomponent supramolecules 4, 7, and 8. To achieve a quantitative insight into such selective multicomponent self-assembly, a computational study has been performed to estimate the energy preference between the homoleptic and heteroleptic systems. To reduce redundancy and maintain accuracy, a simplified model system was used, as shown in Scheme 3, involving only two 90° acceptors, two pyridine (Py), and two benzoate anions (C7H5O2−). Upon heteroleptic coordination (the bottom route in Scheme 3), two heteroleptic molecules cis-Pt(PEt3)2PyC7H5O2+ may be formed, whereas the top route in Scheme 3 results in the formation of homoleptic species cis-Pt(PEt3)2Py22+ and cis-Pt(PEt3)2(C7H5O2)2. Computational simulations (MMFF force field, gas phase, 300 K) for these heteroleptic and homoleptic species were carried out using Maestro and Macromodel. According to MMFF computational results (see Supporting Information), the energy of the system containing two heteroleptic molecules cis-Pt(PEt3)2PyC7H5O2+ is significantly lower than that with two homoleptic species (ΔE = −364.6 kJ/mol), and, therefore, the heteroleptic system is favored.

Well supported by both the experimental data and computational calculations, this selective self-assembly is solely driven by the intrinsic information of the molecular components, without involving a template or extra directing factors such as steric and topological constraints.13-15 Presumably, the different electronic nature of carboxylate (negative) and pyridine (neutral) donor is a major driving factor, which allows for the heteroleptic coordination via charge separation. For example, in the simplified system illustrated in Scheme 3, within the homoleptic complex cis-Pt(PEt3)2Py22+, two coordinated pyridyl moieties may be partially charged22 and result in electrostatic repulsion between them. On the other hand, for the heteroleptic species cis-Pt(PEt3)2PyC7H5O2+, only one pyridyl moiety is coordinated to each Pt(II) center, and, therefore, the charges can be separated and the electrostatic repulsion can be reduced. Thus, the heteroleptic complex carries the lower energy. Additional experimental and computational studies are underway to further characterize the preference for heteroleptic complex formation. Understanding the factors driving these systems will allow optimization of this strategy for the formation of new multi-component assemblies.

Self-assembly via supramolecule-to-supramolecule transformations

Multicomponent, selective self-assembly can be achieved not only by the conventional assembly of individual molecular components, as described above, but also via supramolecule-to-supramolecule transformations. Supramolecular transformations may be defined as a process whereby a supramolecular species alters its structure (and composition) upon suitable external stimulus, such as photo, electrochemical, or chemical signals.18 Here, we describe multicomponent selective self-assembly via supramolecular transformations from homoleptic to heteroleptic systems as shown in Scheme 4. The investigation was carried out by first preparing the homoleptic self-assemblies of 90° Pt(II) acceptor 1 with pyridyl ligands 3, 4, and 5b, and then adding the neutral triangle 12 assembled by 90° Pt(II) acceptor 1 and carboxylate ligand 2. As a result, the pre-assembled homoleptic square 9, truncated tetrahedron 10, and trigonal prism 11 can be entirely transformed to the multicomponent heteroleptic rectangle 4, trigonal prism 7, and tetragonal prism 8b, respectively.

The homoleptic ensembles 9, 10, and 11 as well as the neutral triangle 12 were obtained by mixing the 90° Pt(II) acceptor 1 with pyridyl ligands 3, 4, and 5b and carboxylate donor 2, respectively, in 1:1 (9), 3:2 (10), 2:1 (11), and 1:1 (12) ratios. Each structure was characterized by 31P and 1H multinuclear NMR spectroscopy, ESI mass spectrometry, and PGSE NMR measurements. In the 31P{1H} NMR spectra (Figure 6a-d), only one intense singlet (9: 0.36 ppm; 10: 0.29 ppm; 11: 0.90 ppm; 12: 3.52 ppm) with concomitant 195Pt satellites can be found. Likewise, the 1H NMR spectra (Figure S7,9,11,14 in Supporting Information) show sharp signals assigned to the coordinated pyridyl moieties (e.g. δ = 9.28 ppm, Hα-Py in 9; δ = 9.32 ppm, Hα-Py in 10; δ = 9.75 ppm, Hα-Py in 11) and the carboxylate moieties (δ = 7.74 ppm in 12). These NMR spectral results are in accord with the highly symmetric structure of 9, 10, 11, and 12. ESI mass spectrometry further confirms these assemblies (see Supporting Information). Signals for the [4 + 4] and [6 + 4] self-assembly of 9 and 10 can be found at m/z = 1869.91 [9 – 2OTf]2+, m/z = 1197.08 [9 – 3OTf]3+, m/z = 1818.36 [10 – 3OTf]3+, and m/z = 1326.65 [10 – 4OTf]4+. [6 + 3] Self-assembly of trigonal prism 11 was also supported by observation of isotopically resolved signals at m/z = 2966.57 [11 – 2OTf]2+, m/z = 1408.99 [11 – 4OTf]4+, and m/z = 1097.48 [11 – 5OTf]5+, but those for larger assemblies such as [4 + 4], [5 + 5], and [6 + 6] could not be found, ruling out the formation of larger prisms.23 For the neutral triangle 12, the molecular ion peak for the [3 + 3] self-assembly was found at m/z = 1787.12 [12 + H]+ and m/z = 1809.11 [12 + Na]+, along with the signal for [12 + 2Na]2+at m/z = 916.16. There is no molecular ion peak for [2 + 2] or [4 + 4] ensembles observed in the mass spectrum, excluding formation of a [2 + 2] rectangle or a [4 + 4] square. Furthermore, a PGSE NMR study was also carried out to estimate the size of the trigonal prism 11, and the experimental radius obtained (1.70 ± 0.04 nm) is in good agreement with the computational value of 1.6 nm from MMFF modeling (Figure S13 in Supporting Information).21

Figure 6.

Figure 6

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31P{1H} NMR spectra of the homoleptic self-assemblies 9 (a), 10 (b), and 11 (c), and the neutral triangle 12 (d), as well as the multicomponent rectangle 4 (e), trigonal prism 7 (f), and tetragonal prism 8b (g) obtained via supramolecular transformations.

The transformation of the homoleptic self-assemblies into the heteroleptic assemblies was carried out by the addition of a solution of the above homoleptic product to the neutral triangle 12. After 5 h of heating at 75 °C, a clear solution was obtained and characterized by 31P and 1H multinuclear NMR spectroscopy. In the 31P{1H} NMR spectra (Figure 6e-g), two intense coupled doublet peaks (Figure 6e: 6.63 ppm and 1.08 ppm, 2JP-P = 22.0 Hz for 4; Figure 6f: 6.63 ppm and 1.03 ppm, 2JP-P = 21.4 Hz for 7; Figure 6g: 5.03 ppm and −0.35 ppm, 2JP-P = 21.4 Hz for 8b) with concomitant 195Pt satellites were found, which were in good agreement with those observed in Figure 1 and 3 for the authentic heteroleptic cages. Likewise, in the 1H NMR spectra (Figure S16 in Supporting Information), the signals of the heteroleptic structures obtained by supramolecular transformation match the signals of the authentic ensembles. Peaks corresponding to the starting homoleptic structures were absent in both 31P{1H} and 1H NMR spectra. These NMR data clearly indicate that the homoleptic assemblies have been entirely transformed into the multicomponent heteroleptic structures.

To further investigate the supramolecular transformation process, we carried out a study of the gradual transformation of square 9 to rectangle 4: 10%, 25%, 50%, and 100% of neutral triangle 12 was added to an acetone solution of square 9, and the mixtures were heated at 75 °C for 5 h. The resulting clear solutions were characterized by 31P and 1H multinuclear NMR spectroscopy. As indicated by the 31P{1H} NMR spectra (Figure 7), increasing the amount of 12 resulted in a decrease of the signal (δ = 0.36 ppm) for square 9 and the simultaneous formation of signals around 6.63 ppm and 0.99 ppm, attributable to heteroleptic complexes. A similar result can be observed in 1H NMR spectra (Figure S17 in Supporting Information) by comparing the signals for Hα-Py of the homoleptic assembly (δ = 9.28 ppm) and the heteroleptic complexes (δ = 9.00 ppm). These 31P and 1H NMR spectral results demonstrate the gradual transformation of heteroleptic complexes from the homoleptic species upon addition of the neutral triangle. During the transformation process, in addition to rectangle 4, an intermediate was also observed, indicated by the multiplets around 6.63 ppm and 0.99 ppm in Figure 7b–d. Isotopically resolved signals in the ESI mass spectrum (see Supporting Information) at m/z = 722.56 [M – 3OTf]3+ and 1158.25 [M – 2OTf]2+ suggest this intermediate is formed by the [3 + 2 + 1] assembly of organoplatinum acceptor 1, pyridyl donor 3, and carboxylate ligand 2, and these signals cannot be found in the spectrum of pure rectangle 4, square 9, or neutral triangle 12. The intermediate is formed due to the improper ratio of square and neutral triangle for generating the [4 + 2 + 2] rectangle 4. Once 100% of neutral triangle 12 was added, square 9 can be fully altered to rectangle 4 as indicated by Figure 7e and Figure S17e.

Figure 7.

Figure 7

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31P{1H} NMR spectra for mixtures of square 9 upon addition of 0% (a), 10% (b), 25% (c), 50% (d), and 100% (e) of neutral triangle 12.

The Host-Guest Properties of the Multicomponent Structure

To explore potential applications of such multicomponent structures, a host-guest study was carried out as shown in Scheme 5: a multicomponent porphyrin cage soluble in aqueous acetone solution was prepared by the method described above, and then used as a host for encapsulating the aromatic guest, triphenylene. The multicomponent porphyrin cage 14 was prepared by mixing cis-Pt(PMe3)(OTf)2 (13) with tetrakis(4-pyridyl) porphyrin (6b) and sodium terephthalate (2) in a 4:1:2 ratio in an aqueous acetone solution (v/v 1:1). After heating at 85 °C for 24 h, a clear, purple solution was obtained. The 31P{1H} NMR spectrum (Figure S19 in the Supporting Information) of the solution shows two doublets at −24.3 ppm and −30.3 ppm of equal intensity with concomitant 195Pt satellites. The 1H NMR spectrum (Figure 8a), shows signals corresponding to coordinated pyridyl and carboxylate moieties, identified at 9.22 ppm (Hα-Py), 8.28 ppm (Hβ-Py), and 7.95 ppm (Hphenyl-2). The NMR spectral evidence clearly indicates that a heteroleptic ensemble was predominantly formed in the solution. Isotopically resolved signals in the ESI-MS spectrum at m/z = 1806.24 ([14 – 3OTf]3+) and m/z = 1317.69 ([14 – 4OTf]4+) (Figure S20 in the Supporting Information) further support that the heteroleptic species is the multicomponent porphyrin cage 14.

Scheme 5.

Scheme 5

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Graphical representation of self-assembly of multicomponent porphyrin cage 14 by cis-Pt(PMe3)2(OTf)2 13 with carboxylate 2 and pyridyl ligands 6b and encapsulation of triphenylene TP.

Figure 8.

Figure 8

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Partial 1H NMR spectra (300 MHz, acetone-d6/D2O = 1:1) of pure 14 (a), TP (b), and the host-guest mixture (c).

The host-guest properties of 14 were studied by the addition of excess triphenylene (TP) in aqueous acetone solution to the cage. After heating at 85 °C for 16 h, the encapsulated complex was present in the mixture. Two doublets at −24.3 ppm −30.3 ppm with concomitant 195Pt satellites in the 31P{1H} NMR spectrum (see Supporting Information) and the identifiable peaks (δ = 9.22 ppm for Hα-Py, δ = 8.28 ppm for Hβ-Py, and δ = 7.95 ppm for Hphenyl-2) in the 1H NMR spectra (Figure 8c) indicated that the coordinative porphyrin cage structure was retained. By comparing the 1H NMR spectra of pure 14 and TP with the mixture (Figure 8), shifted signals were observed at 8.11 ppm (Δδ = −0.54 ppm, Htriphenylene) and 7.26 ppm (Δδ = −0.38 ppm, Htriphenylene), indicating that the triphenylene TP was encapsulated in the porphyrin cage, 14. The ESI mass spectra (Figure 9), showed isotopically resolved signals at m/z = 1373.91 [14·TP – 4OTf]4+ and m/z = 1069.72 [14·TP – 5OTf]5+ confirming that one triphneylene molecule TP was encapsulated in the cage of 14. Integration of the proton NMR signals indicated that 27% of cage 14 contained guest molecules under these conditions.

Figure 9.

Figure 9

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Calculated (blue, top) and Experimental (red, bottom) ESI mass spectrum of the encapsulated complex 14·TP.

A computational simulation was used to gain insight about the structural features of the encapsulated complex 14·TP. A molecular dynamics simulation using MM2* force fields at 300K in the aqueous phase was used to equilibrate the complex, and the output of the simulation was then minimized to full convergence. As shown in Figure 10, one triphenylene molecule TP is trapped within the cavity of 14, and the distance between the guest and porphyrin faces is about 3.8 Å.

Figure 10.

Figure 10

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Varied views of the computational model (MM2*) of the encapsulated complex 14·TP.

Conclusion

We report here a facile and very efficient approach for the selective construction of well-defined multicomponent 2-D and 3-D supramolecular structures of various motifs. Upon combination of a 90° Pt(II) acceptor and a carboxylate ligand with appropriate pyridyl donors in a proper ratio, coordination-driven self-assembly allows for the selective formation of a multicomponent supramolecular rectangle and prisms. These multicomponent complexes can also be obtained by a novel supramolecule-to-supramolecule transformation from homoleptic assemblies. Characterization via multinuclear (31P and 1H) NMR spectroscopy clearly reveals the heteroleptic coordination nature of these assembled supramolecules, as well as their high structural symmetry. ESI mass spectrometry and PGSE NMR measurements, together with computational simulations, further identify the composition and size of these multicomponent assemblies. Presumably, such multicomponent, selective self-assembly processes are directed, in part, by a charge separation effect, whereby a negative carboxylate ligand and a neutral pyridyl donor favors a heteroleptic motif upon coordination with Pt(II) centers. These cage structures show a unique 3-D nanoscale pore, and preliminary studies indicate the nano-cavity is able to encapsulate triphenylene. These selectively self-assembled multicomponent supramolecules can be further developed into functionalized scaffolds via pre/post-modifications, which is currently under investigation.

Experimental Section

Methods and Materials

Molecular building blocks 124, 325, 526, and 6a17b were prepared according to literature procedures. Carboxylate ligand 2 was prepared by neutralization of terephthalic acid with 2 equiv. NaOH. All other reagents were purchased from Aldrich or Alfa and used without further purification. Deuterated solvents were purchased from Cambridge Isotope Laboratory (Andover, MA). Multinuclear (31P and 1H) NMR spectra were recorded on a Varian Unity 300 spectrometer, and PGSE NMR data were obtained on an Inova 500 MHz spectrometer. Mass spectra were recorded on a Micromass Quattro II triple-quadrupole mass spectrometer using electrospray ionization with a MassLynx operating system. Element analysis was performed by Atlantic Microlab (Norcross, GA).

General procedure for selective self-assembly

Cis-Pt(PEt3)2(OTf)2 1, carboxylate ligand 2, and various pyridyl donors were placed in a 2-dram vial, followed by addition of D2O (0.2 mL) and Acetone-d6 (0.8 mL). After 3 h of heating at 75 °C, all solvent was removed by N2 flow, and then dried under vacuum. Acetone-d6 (0.7 mL) was then added into each mixture. A clear solution was obtained after an additional 5 h of heating at 75 °C. The resulted multicomponent supramolecules were isolated via precipitation by addition of Et2O or KPF6.

Synthesis and Characteristics of 4

Reaction scale: Cis-Pt(PEt3)2(OTf)2 1 (5.37 mg, 7.36 μmol), carboxylate ligand 2 (0.77 mg, 3.7 μmol), and ditopic pyridyl ligand 3 (1.03 mg, 3.68 μmol). Yield: 90%. MS (ESI) calcd for [M – 2OTf]2+ m/z 1455.85, found 1455.69; calcd for [M – 3OTf]3+ m/z 920.92, found 920.88. 1H NMR (Acetone-d6, 300MHz) δ 9.00 (s, 8H, Hα-Py), 7.74 (m, 16H, Hβ-Py and Hphenyl-Py), 7,66 (s, 8H, Hphneyl), 1.92 (m, 48H, PCH2CH3), 1.21 (m, 72H, PCH2CH3). 31P{1H} NMR (Acetone-d6, 121.4 MHz) δ 6.60 (d, 2JP-P = 22.0 Hz, 195Pt satellites, 1JPt-P = 3242 Hz), 1.06 (d, 2JP-P = 22.0 Hz, 195Pt satellites, 1JPt-P = 3427 Hz). Anal. Calcd for C108H152F12N4O20P8Pt4S4: C, 40.40; H, 4.77; N, 1.74. Found: C, 40.04; H, 4.70; N, 1.71.

Synthesis and Characteristics of 7

Reaction scale: Cis-Pt(PEt3)2(OTf)2 1 (4.88 mg, 6.69 μmol), carboxylate ligand 2 (0.71 mg, 3.4 μmol), and tritopic pyridyl ligand 5 (0.84 mg, 2.2 μmol). Yield: 91%. MS (ESI) calcd for [M – 2OTf]2+ m/z 2218.98, found 2218.76; calcd for [M – 3OTf]3+ m/z 1429.67, found 1429.59. 1H NMR (Acetone-d6, 300MHz) δ 9.02 (s, 12H, Hα-Py), 7,88 (s, 12H, Hphneyl-Py), 7.75 (m, 24H, Hβ-Py and Hphenyl), 1.89 (m, 72H, PCH2CH3), 1.20 (m, 108H, PCH2CH3). 31P{1H} NMR (Acetone-d6, 121.4 MHz) δ 6.56 (d, 2JP-P = 22.0 Hz, 195Pt satellites, 1JPt-P = 3227 Hz), 1.01 (d, 2JP-P = 22.0 Hz, 195Pt satellites, 1JPt-P = 3404 Hz). Anal. Calcd for C156H222F18N6O30P12Pt6S6: C, 39.55; H, 4.72; N, 1.77. Found: C, 39.92; H, 4.54; N, 1.79.

Synthesis and Characteristics of 8a

Reaction scale: Cis-Pt(PEt3)2(OTf)2 1 (5.22 mg, 7.16 μmol), carboxylate ligand 2 (0.75 mg, 3.6 μmol), and tetratopic pyridyl ligand 6a (1.14 mg, 1.78 μmol). Yield: 96%. MS (ESI) calcd for [M – 3PF6]3+ m/z 2037.86, found 2037.75; calcd for [M – 5PF6]5+ m/z 1164.51, found 1164.50. 1H NMR (Acetone-d6, 300MHz) δ 8.87 (s, 16H, Hα-Py), 7,89 (d, J = 6 Hz, 16H, Hα-phneylPy), 7.60 (m, 32H, Hβ-Py and Hphenyl), 7,20 (d, J = 6 Hz, 16H, Hβ-phneyl-Py), 1.89 (m, 96H, PCH2CH3), 1.17 (m, 144H, PCH2CH3). 31P{1H} NMR (Acetone-d6, 121.4 MHz) δ 5.88 (d, 2JP-P = 21.4 Hz, 195Pt satellites, 1JPt-P = 3270 Hz), 1.08 (d, 2JP-P = 21.4 Hz, 195Pt satellites, 1JPt-P = 3448 Hz). Anal. Calcd for C220H320F48N8O16P24Pt8: C, 40.35; H, 4.93; N, 1.71. Found: C, 40.71; H, 5.08; N, 1.74.

Synthesis and Characteristics of 8b

Reaction scale: Cis-Pt(PEt3)2(OTf)2 1 (4.52 mg, 6.10 μmol), carboxylate ligand 2 (0.63 mg, 3.0 μmol), and tetratopic pyridyl ligand 6b (0.94 mg, 1.5 μmol). Yield: 95%. MS (ESI) calcd for [M – 3PF6]3+ m/z 2022.83, found 2022.71; calcd for [M – 5PF6]5+ m/z 1155.71, found 1155.62. 1H NMR (CD3NO2, 300MHz) δ 9.27 (m, 24H, Hα-Py and HPyrrole), 8.34 (d, J = 5.7 Hz, 16H, Hβ-Py), 8.08 (s, 16H, Hphneyl), 7.12 (s, 8H, HPyrrole), 2.24 (m, 96H, PCH2CH3), 1.39 (m, 144H, PCH2CH3). −3.28 (s, 4H, HN-H). 31P{1H} NMR (CD3NO2, 121.4 MHz) δ 5.07 (d, 2JP-P = 21.4 Hz, 195Pt satellites, 1JPt-P = 3270 Hz), −0.34 (d, 2JP-P = 21.4 Hz, 195Pt satellites, 1JPt-P = 3462 Hz). Anal. Calcd for C208H308F48N16O16P24Pt8: C, 38.41; H, 4.77; N, 3.45. Found: C, 38.07; H, 4.92; N, 3.41.

Self-Assembly of 9

Cis-Pt(PEt3)2(OTf)2 1 (5.07 mg, 6.95 μmol) and ditopic pyridyl ligand 3 (1.95 mg, 6.96 μmol) were placed in a 2-dram vial, followed by addition of 0.7 mL Acetone-d6, which was then sealed with Teflon tape and immersed in an oil bath at 70 °C for 2 h. Solid product was obtained by removing the solvent under vacuum. Yield: 97%. MS (ESI) calcd for [M – 2OTf]2+ m/z 1869.84, found 1869.91; calcd for [M – 3OTf]3+ m/z 1197.25, found 1197.08. 1H NMR (Acetone-d6, 300MHz) δ 9.28 (d, J = 4.8 Hz, 16H, Hα-Py), 7.79 (d, J = 6.0 Hz, 16H, Hβ-Py), 7.69 (s, 16H, Hphneyl-Py), 2.08 (m, 48H, PCH2CH3), 1.29 (m, 72H, PCH2CH3). 31P{1H} NMR (Acetone-d6, 121.4 MHz) δ 0.36 (s, 195Pt satellites, 1JPt-P = 3099 Hz). Anal. Calcd for C136H168F24N8O24P8Pt4S8: C, 40.44; H, 4.19; N, 2.77. Found: C, 40.65; H, 4.21; N, 2.64.

Self-Assembly of 10

Cis-Pt(PEt3)2(OTf)2 1 (4.51 mg, 6.18 μmol) and tritopic pyridyl ligand 5 (1.55 mg, 4.06 μmol) were placed in a 2-dram vial, followed by addition of 0.7 mL Acetone-d6, which was then sealed with Teflon tape and immersed in an oil bath at 70 °C for 2 h. Solid product was obtained by removing the solvent under vacuum. Yield: 98%. MS (ESI) calcd for [M – 3OTf]3+ m/z 1818.32, found 1818.36; calcd for [M – 4OTf]4+ m/z 1326.75, found 1326.65. 1H NMR (Acetone-d6, 300MHz) δ 9.32 (d, J = 4.8 Hz, 24H, Hα-Py), 8.04 (s, 12H, Hphneyl-Py), 7.83 (d, J = 6.3 Hz, 24H, Hβ-Py), 2.09 (m, 72H, PCH2CH3), 1.29 (m, 108H, PCH2CH3). 31P{1H} NMR (Acetone-d6, 121.4 MHz) δ 0.29 (s, 195Pt satellites, 1JPt-P = 3083 Hz). Anal. Calcd for C192H240F36N12O36P12Pt6S12: C, 39.07; H, 4.10; N, 2.85. Found: C, 39.25; H, 4.18; N, 2.81.

Self-Assembly of 11

To a 1.2 mL CD2Cl2 suspension of tetratopic pyridyl ligand 6b (2.46 mg, 3.97 μmol) was added a 0.4 mL CD3NO2 solution of Cis-Pt(PEt3)2(OTf)2 1 (5.89 mg, 8.07 μmol), drop by drop, with continuous stirring (5 min). The reaction mixture was stirred at room temperature for 1 h, and then heated up to 70 °C for overnight. The solution was evaporated to dryness, and the product was collected. Yield: 98%. MS (ESI) calcd for [M – 2OTf]2+ m/z 2966.54, found 2966.57; calcd for [M – 4OTf]4+ m/z 1409.04, found 1408.99; calcd for [M – 5OTf]5+ m/z 1097.44, found 1097.48. 1H NMR (CD2Cl2/CD3NO2: 3/1, 300MHz) δ 9.75 (dd, J1 = 4.8 Hz and J2 = 36 Hz, 24H, Hα-Py), 8.98 (m, 36H, HβPy and HPyrrole), 8.51 (s, 12H, HPyrrole), 2.32 (m, 72H, PCH2CH3), 1.60 (m, 108H, PCH2CH3). 31P{1H} NMR (CD2Cl2/CD3NO2: 3/1, 121.4 MHz) δ 0.90 (s, 195Pt satellites, 1JPt-P = 3070 Hz). Anal. Calcd for C204H258F36N24O36P12Pt6S12: C, 39.31; H, 4.17; N, 5.39. Found: C, 39.83; H, 4.35; N, 5.16.

Self-Assembly of 12

Cis-Pt(PEt3)2(OTf)2 1 (2.12 mg, 2.91 μmol) and carboxylate ligand 2 (0.61 mg, 2.9 μmol) were placed in a 2-dram vial, followed by addition of 0.08 mL H2O and 0.8 Acetone, which was then sealed with Teflon tape and immersed in an oil bath at 70 °C for 2 h. The solvent was then removed by N2 flow, and the solid mixture was dried under vacuum. 0.6 mL Acetone-d6 was added to the dried mixture and after 4 h of heating at 70 °C, the neutral triangle was formed. MS (ESI) calcd for [M + H]+ m/z 1787.38, found 1787.12; calcd for [M + Na]+ m/z 1809.47, found 1809.11; calcd for [M + 2Na]2+ m/z 916.23, found 916.16. 1H NMR (Acetone-d6, 300MHz) δ 7.74 (s, 12H, HPhenyl), 2.00 (m, 36H, PCH2CH3), 1.23 (m, 54H, PCH2CH3). 31P{1H} NMR (Acetone-d6, 121.4 MHz) δ 3.52 (s, 195Pt satellites, 1JPt-P = 3619 Hz).

Self-Assembly of 14

Cis-Pt(PMe3)2(OTf)2 13 (3.92 mg, 5.38 μmol), carboxylate ligand 2 (0.56 mg, 2.67 μmol), and tetratopic pyridyl ligand 6b (0.82 mg, 1.3 μmol) were placed in a 2-dram vial, followed by addition of 0.6 mL D2O and 0.6 Acetone-d6, which was then sealed with Teflon tape and immersed in an oil bath at 85 °C for 24 h. The multicomponent porphyrin cage 14 was formed. The solid product can be isolated by ion exchange with KPF6. Yield: 95%. MS (ESI) calcd for [M – 4OTf]4+ m/z 1317.69, found 1317.69; calcd for [M – 3OTf]3+ m/z 1806.23, found 1806.24. 1H NMR (D2O/Acetone-d6: 1/1, 300MHz) δ 9.22 (m, 24H, Hα-Py and HPyrrole), 8.28 (d, J = 5.7 Hz, 16H, Hβ-Py), 7.95 (s, 16H, Hphneyl), 6.96 (s, 8H, HPyrrole), 1.95 (d, J = 12 Hz, 72H, PCH3), 1.80 (d, J = 12 Hz, 72H, PCH3). 31P{1H} NMR (D2O/Acetone-d6: 1/1, 121.4 MHz) δ −24.3 (d, 2JP-P = 23.8 Hz, 195Pt satellites, 1JPt-P = 3324 Hz), −30.3 (d, 2JP-P = 23.8 Hz, 195Pt satellites, 1JPt-P = 3496 Hz). Anal. Calcd for C160H212F48N16O16P24Pt8: C, 32.95; H, 3.66; N, 3.84. Found: C, 33.16; H, 3.60; N, 3.74.

Supplementary Material

1_si_001

Figure 4.

Figure 4

Open in a new tab

Full ESI mass spectrum of the tetragonal prism 8a.

Acknowledgment

P.J.S. thanks the NIH (Grant GM-057052) for financial support.

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Supplementary Materials

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NIHMS251001-supplement-1_si_001.pdf (979.9KB, pdf)

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